Tablet computer systems, electronic book (e-book) readers, smart phones, and other types of portable devices are increasingly popular. These types of devices have features in common, such as high resolution touchscreens that provide an easy-to-use and intuitive user interface and allow users to interact directly with what is being displayed.
In an e-reader, for example, a page of an e-book is rendered and displayed. The electronic version of the page that is displayed looks very much like the conventional non-electronic version of the page.
One advantage that an e-book has over a conventional non-electronic book is that characteristics of the electronic page can be changed to satisfy a user's preferences considering factors such as ambient light conditions. For example, if the user prefers that the display be brighter or less bright, then the user can adjust the brightness up and down. Brightness can be readily adjusted by, for example, placing two fingers on or near the touchscreen, and then moving (swiping) the fingers up to increase brightness or down to decrease brightness.
Some types of devices, including some e-book readers, allow the user to choose between a first mode, usually known as day mode, and a second mode, usually known as night mode. In day mode, the text is a darker color (usually black) and the background is a lighter color (usually white or nearly white); in night mode, the text is a lighter color (such as white or nearly white) and the background is a darker color (usually black). The level of brightness can be adjusted whether in day mode or in night mode as described above.